1. Field of the Invention
The present invention relates to classification of wet coals for production of high-quality coke suitable for use in a blast furnace.
2. Description of the Prior Art
Conventionally, for production of coke for a blast furnace, the material coals (strongly coking coal and weakly coking coal) are mixed to obtain a mixture which is crushed and mixed and then charged into a coke oven for dry distillation.
However, in recent years it has been increasingly difficult to secure ample amounts of the strongly coking coal and various technical developments have been made for the purpose for expanding the applicability of various grades of coals, among which a classification-selective crushing method is well known.
Coals have different degrees of grindability depending on their constituents. Therefore, if they are crushed all together as conventionally done, relatively larger proportions of hard components, such as hard coal and durain having a lower coking property are likely to be distributed in the resultant larger particles and these hard components form the nucleus of cracking in the coke during dry distillation, thus resulting in brittle cokes. Meanwhile, the soft components, such as vitrain and clarain, are crushed unnecessarily.
Therefore, if coal particles rich in durain components which are hard to be crushed are selectively and repeatedly crushed and mixed together appropriately, it is possible to obtain uniform coal charges and the resultant cokes are improved in their quality, thus making possible to decrease the proportion of strongly coking coals to be mixed in the charge.
However, for classification and selective crushing of the material coals, it is necessary to classify the coals first. In this case, when the material coals contain 6% or less moisture, they can be classified by an ordinary vibration screen, but when their moisture contents are more than about 6%, they cannot be satisfactorily classified because they adhere and close the screen meshes. This is the reason why it has been rarely practised to classify and selectively crush the wet coals directly without pretreatments. As pretreatments for enabling the classification, various methods have been proposed, and for example, as disclosed in the Japanese Patent Publication No. Sho 48-29601, a method is known, in which a coal drying machine is provided for drying the coals to a moisture content ranging from 4 to 6% in advance to the screen device or a method is known, in which an electrically heated screen device is used.
However, all of the above conventional arts have disadvantages that a large capacity of the drying equipment is required for drying a large amount of the material coals to a moisture content less than 6%, that a large amount of energy is consumed, or that a short-time drying and careful considerations against public pollution are required, and these conventional arts have been confronted with by a problem that if the moisture content of material coals cannot be uniformly lowered, the classification efficiency will be critically lowered.
The present inventors have made search for a classifier which can satisfactorily classify the wet coals, but found that none of the conventional classifiers can classify the wet coals directly.
Then the present inventors have made various extensive experiments on applicability of a rotary type of classifier to classification of wet coals. This type of classifier shows a good classification efficiency and is of a compact size and almost free from dust generation. The results of the experiments have revealed that the classification of wet coals can be continuously and satisfactorily performed if a high-pressure gas is blown to the screen meshes under specific conditions.
The material coals for coke production which are available in Japan usually contain more than 6% moisture, depending on their origins, and their particle size distribution is that -3 mm mesh particle ranges from 30 to 80%.
According to the results of our experiments, the adhesion to the screen meshes of the above type of material coals differ depending on the moisture contents and the proportions of the fine particles. For example, when the proportion of the fine particles is constant, the adhesion to the screen meshes increases as the moisture content is raised, showing a peak at a moisture content from 11 to 12%, and thereafter, lowers and when the moisture content exceeds 14%, the classification becomes possible, but this level of moisture content is undesirable for charging in the coke oven. For charges in the coke oven, a lower moisture content is more desirable.